438,798 research outputs found
On the Size of Structures in the Solar Corona
Fine-scale structure in the corona appears not to be well resolved by current
imaging instruments. Assuming this to be true offers a simple geometric
explanation for several current puzzles in coronal physics, including: the
apparent uniform cross-section of bright threadlike structures in the corona;
the low EUV contrast (long apparent scale height) between the top and bottom of
active region loops; and the inconsistency between loop densities derived by
spectral and photometric means. Treating coronal loops as a mixture of diffuse
background and very dense, unresolved filamentary structures address these
problems with a combination of high plasma density within the structures, which
greatly increases the emissivity of the structures, and geometric effects that
attenuate the apparent brightness of the feature at low altitudes. It also
suggests a possible explanation for both the surprisingly high contrast of EUV
coronal loops against the coronal background, and the uniform ``typical''
height of the bright portion of the corona (about 0.3 solar radii) in full-disk
EUV images. Some ramifications of this picture are discussed, including an
estimate (10-100 km) of the fundamental scale of strong heating events in the
corona.Comment: To appear in APJ, June 2007; as accepted Feb 200
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
High Resolution Helioseismic Imaging of Subsurface Structures and Flows of A Solar Active Region Observed by Hinode
We analyze a solar active region observed by the Hinode CaII H line using the
time-distance helioseismology technique, and infer wave-speed perturbation
structures and flow fields beneath the active region with a high spatial
resolution. The general subsurface wave-speed structure is similar to the
previous results obtained from SOHO/MDI observations. The general subsurface
flow structure is also similar, and the downward flows beneath the sunspot and
the mass circulations around the sunspot are clearly resolved. Below the
sunspot, some organized divergent flow cells are observed, and these structures
may indicate the existence of mesoscale convective motions. Near the light
bridge inside the sunspot, hotter plasma is found beneath, and flows divergent
from this area are observed. The Hinode data also allow us to investigate
potential uncertainties caused by the use of phase-speed filter for short
travel distances. Comparing the measurements with and without the phase-speed
filtering, we find out that inside the sunspot, mean acoustic travel times are
in basic agreement, but the values are underestimated by a factor of 20-40%
inside the sunspot umbra for measurements with the filtering. The initial
acoustic tomography results from Hinode show a great potential of using
high-resolution observations for probing the internal structure and dynamics of
sunspots.Comment: accepted for publication in Ap
Towards a Realistic, Data-Driven Thermodynamic MHD Model of the Global Solar Corona
In this work we describe our implementation of a thermodynamic energy
equation into the global corona model of the Space Weather Modeling Framework
(SWMF), and its development into the new Lower Corona (LC) model. This work
includes the integration of the additional energy transport terms of coronal
heating, electron heat conduction, and optically thin radiative cooling into
the governing magnetohydrodynamic (MHD) energy equation. We examine two
different boundary conditions using this model; one set in the upper transition
region (the Radiative Energy Balance model), as well as a uniform chromospheric
condition where the transition region can be modeled in its entirety. Via
observation synthesis from model results and the subsequent comparison to full
sun extreme ultraviolet (EUV) and soft X-Ray observations of Carrington
Rotation (CR) 1913 centered on Aug 27, 1996, we demonstrate the need for these
additional considerations when using global MHD models to describe the unique
conditions in the low corona. Through multiple simulations we examine ability
of the LC model to asses and discriminate between coronal heating models, and
find that a relative simple empirical heating model is adequate in reproducing
structures observed in the low corona. We show that the interplay between
coronal heating and electron heat conduction provides significant feedback onto
the 3D magnetic topology in the low corona as compared to a potential field
extrapolation, and that this feedback is largely dependent on the amount of
mechanical energy introduced into the corona.Comment: 17 pages, 11 figures, Submitted to ApJ on 12/08/200
Star formation efficiency in the Barred Spiral Galaxy NGC 4303
We present new CO(J=1-0) observations of the barred galaxy NGC 4303
using the Nobeyama 45m telescope (NRO45) and the Combined Array for Research in
Millimeter-wave Astronomy (CARMA). The H images of barred spiral
galaxies often show active star formation in spiral arms, but less so in bars.
We quantify the difference by measuring star formation rate and efficiency at a
scale where local star formation is spatially resolved. Our CO map covers the
central 2\farcm3 region of the galaxy; the combination of NRO45 and CARMA
provides a high fidelity image, enabling accurate measurements of molecular gas
surface density. We find that star formation rate and efficiency are twice as
high in the spiral arms as in the bar. We discuss this difference in the
context of the Kennicutt-Schimidt (KS) law, which indicates a constant star
formation rate at a given gas surface density. The KS law breaks down at our
native resolution ( 250 pc), and substantial smoothing (to 500 pc) is
necessary to reproduce the KS law, although with greater scatter.Comment: 17 pages, 10 figures, published by ApJ;
http://adsabs.harvard.edu/abs/2010ApJ...721..383
Decorrelation Times of Photospheric Fields and Flows
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3 \arcsec), high-cadence ( min) line-of-sight
magnetograms of NOAA active region (AR) 10930 recorded by the Narrowband Filter
Imager (NFI) of the Solar Optical Telescope (SOT) aboard the {\em Hinode}
satellite over 12--13 December 2006. To baseline the timescales of flow
evolution, we also autocorrelated the magnetograms, at several spatial
binnings, to characterize the lifetimes of active region magnetic structures
versus spatial scale. Autocorrelation of flow maps can be used to optimize
tracking parameters, to understand tracking algorithms' susceptibility to
noise, and to estimate flow lifetimes. Tracking parameters varied include: time
interval between magnetogram pairs tracked, spatial binning applied
to the magnetograms, and windowing parameter used in FLCT. Flow
structures vary over a range of spatial and temporal scales (including
unresolved scales), so tracked flows represent a local average of the flow over
a particular range of space and time. We define flow lifetime to be the flow
decorrelation time, . For , tracking results represent
the average velocity over one or more flow lifetimes. We analyze lifetimes of
flow components, divergences, and curls as functions of magnetic field strength
and spatial scale. We find a significant trend of increasing lifetimes of flow
components, divergences, and curls with field strength, consistent with Lorentz
forces partially governing flows in the active photosphere, as well as strong
trends of increasing flow lifetime and decreasing magnitudes with increases in
both spatial scale and .Comment: 48 pages, 20 figures, submitted to the Astrophysical Journal;
full-resolution images in manuscript (8MB) at
http://solarmuri.ssl.berkeley.edu/~welsch/public/manuscripts/flow_lifetimes_v2.pd
Forest disturbance and recovery: A general review in the context of spaceborne remote sensing of impacts on aboveground biomass and canopy structure
Abrupt forest disturbances generating gaps \u3e0.001 km2 impact roughly 0.4–0.7 million km2a−1. Fire, windstorms, logging, and shifting cultivation are dominant disturbances; minor contributors are land conversion, flooding, landslides, and avalanches. All can have substantial impacts on canopy biomass and structure. Quantifying disturbance location, extent, severity, and the fate of disturbed biomass will improve carbon budget estimates and lead to better initialization, parameterization, and/or testing of forest carbon cycle models. Spaceborne remote sensing maps large-scale forest disturbance occurrence, location, and extent, particularly with moderate- and fine-scale resolution passive optical/near-infrared (NIR) instruments. High-resolution remote sensing (e.g., ∼1 m passive optical/NIR, or small footprint lidar) can map crown geometry and gaps, but has rarely been systematically applied to study small-scale disturbance and natural mortality gap dynamics over large regions. Reducing uncertainty in disturbance and recovery impacts on global forest carbon balance requires quantification of (1) predisturbance forest biomass; (2) disturbance impact on standing biomass and its fate; and (3) rate of biomass accumulation during recovery. Active remote sensing data (e.g., lidar, radar) are more directly indicative of canopy biomass and many structural properties than passive instrument data; a new generation of instruments designed to generate global coverage/sampling of canopy biomass and structure can improve our ability to quantify the carbon balance of Earth\u27s forests. Generating a high-quality quantitative assessment of disturbance impacts on canopy biomass and structure with spaceborne remote sensing requires comprehensive, well designed, and well coordinated field programs collecting high-quality ground-based data and linkages to dynamical models that can use this information
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